Metallisation of different features such as gate structures and circuitry for thin film transistor (TFT) based flat panel displays is achieved by depositing metals such as aluminium, molybdenum, chromium and titanium by vapour phase deposition methods. Increasing panel dimensions require a higher electrical conductivity of the TFT lines to maintain very short pixel response times and avoid motion blur effects in fast moving image sequences.
Due to its low specific resistance copper is a suitable candidate to replace other metals like the ones mentioned before as TFT line material. Sputtering of copper is the conventional method. However, sputtering copper becomes increasingly more problematic as soon as the required film thickness exceeds 1 μm. Sputtered copper films typically exhibit a considerable amount of internal stress which eventually can cause bending or even cracking of the underlying glass substrate. Sputtering thick copper layers also suffers from significant material loss due to comparably low sputter yields. Finally, increased maintenance efforts for cleaning the sputter chambers can lead to prolonged idle times and limit the overall process productivity.
A method for electroless deposition of copper onto substrates for flat panel display applications is disclosed by S. Fang et al. in “Highly Adhesive Copper Wiring for FPD using Inkjet Printed Catalyst and Neutral Electroless Deposition” (IDW '07—Proceedings of the 14th International Display Workshops (2007), Volume 2, pp. 713-714). Here, the substrate is activated prior to electroless metal deposition with a catalytic ink deposited by ink jet printing.
Another method for depositing copper by electroless plating onto a layer of molybdenum is disclosed by H. Ning et al. in “The Feasibility of Cu Plating Technology in LCD” (Proceedings of ASIA Display 2007 AD'07 Shanghai, Mar. 12-16, 2007). Said method utilises a patterned photo resist to selectively deposit copper.